Any claim of amazing advances like a quantum radar need to be taken with a pinch of salt. With regards to detection via third party radio transmissions, what y

Any claim of amazing advances like a quantum radar need to be taken with a pinch of salt. With regards to detection via third party radio transmissions, what you're referring to is generally called passive radar [0]. It's actually not a new concept, in that the very first radars were bistatic, with the receiver and transmitter in different locations, and the Watts's earliest prototypes relied on existing BBC shortwave radio transmitters to provide the RF energy. In the 1950s there were some experiments with bistatic radars using scattered RF energy, but the computing power didn't really exist to make them viable. What makes the present generation of passive radar tech different is that modern digital signal processing systems are able to better distinguish sources of RF energy, filter out noise, etc in a way that makes not only detection but also tracking possible. There has also been a massive increase in the number and type of transmitters over the past two decades, with cell towers in particular providing a good source of RF energy. There are still some issues with the approach, in that you're highly dependent on the dispersion and geometry of commercial radio signals, low-observable aircraft are still difficult to detect so it's not a guaranteed solution for those, most systems can't ascertain height, only lat lons, and it's easy for an enemy to destroy enough commercial transmitters to fatally degrade the system. For this reason most of the passive radar systems on offer are being sold as complements to existing civilian ATC radar systems, usually as gap filler systems to enhance radar coverage in areas where it wouldn't make financial sense to deploy a full primary radar. The approach is interesting though and there's a fair bit of research going into it, so we'll probably see much more capable systems before long. [0]: https://en.wikipedia.org/wiki/Passive_radar https://en.wikipedia.org/wiki/Passive_radar